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Treadmill testing

Initial prototype testing began outside an MRI room to match performance to established speed and gradient combinations. We expected belt speed and elevation to stabilize within ± 5% of required values. Sensors and software interlocks also included component maximum speed limits and other safety concerns.

Testing was conducted to ensure that the MRI environment did not influence the operation of the treadmill and controls and that the components of the treadmill could be safely located within the MRI’s magnetic field. A 0.7 Tesla permanent magnet was used to test all individual components of the treadmill for magnetic attraction. The completed treadmill assembly was also tested for magnetic attraction.

Additionally, we tested the machine to ensure it would not influence the quality of images recorded during diagnostic tests. A series of standardized tests was conducted to measure image quality comparing the image quality with no treadmill in the MRI room, to a fully connected, stationary treadmill located close to the magnet, and, finally, to an operating treadmill.

Water as a hydraulic fluid Water is the fluid of choice primarily because the hydraulic motor was designed specifically for use with water. Water has many positive attributes as a hydraulic fluid, but also some challenges. Water is inexpensive and readily available. Its viscosity is significantly lower than oil’s, so water can transmit comparable power with lower pressure differentials, resulting in potentially higher overall efficiency. Even though leakage has not been an issue, water leaks in a hospital setting are less dangerous and easier to clean up than oil leaks are.

Of course, these advantages are offset by some disadvantages. Water’s low viscosity means it flows more readily through small clearances typically found in hydraulic components. Therefore, pumps and motors generally exhibit higher internal leakage, which translates to lower volumetric efficiency.

Unlike oil, water does not readily establish full-film (hydrodynamic) lubrication. Water also can be corrosive to metals and cause erosive wear. Water can be breeding grounds for bacteria and other micro-organisms. Antimicrobial agents can be added to the water to inhibit microbial growth, but we chose, instead, to install an ultraviolet sterilizer in the return flow piping to reduce microbial growth. This does not eliminate microbial growth, so periodic water and filter changes are required.

The current state The initial treadmill prototype served its intended function by meeting performance goals and providing a platform suitable for clinical research on a limited scale. Many minor changes to hardware and software were required to meet specifications and provide reliability.

Based on the success of the initial prototype, a next generation treadmill design has been built to conduct a multicenter clinical trial. Attention to details of clinical ergonomics, software convenience, enhanced performance, and improved serviceability were incorporated into the new design.

Why so close?

Eric Foster is COO of EXCMR Inc., Columbus, the company that designs and manufactures the water hydraulic treadmill. He explains why the treadmill must be so close to the MRI machine:

Heart disease affects more than 70 million Americans and is the leading cause of death in the United States. Two-thirds of these deaths occur without detection of the disease. Treadmill exercise stress testing, combined with cardiac imaging, has become the leading method of detection and treatment of heart disease. In these exams, images of the heart are obtained immediately following a treadmill stress test and compared with baseline images of the heart at rest. This lets doctors observe any abnormalities that develop as the heart demands more blood during physical stress.

These exams are currently performed with either nuclear or ultrasound imaging. Both suffer from technical limitations, and nuclear imaging involves significant radiation exposure. Magnetic Resonance Imaging (MRI) can provide a complete cardiac imaging evaluation that is superior to existing techniques. However, the combination of exercise stress with MRI has not previously been feasible because traditional treadmills cannot be safely placed in the same room as the MRI machine.

Imaging must begin immediately after exercise (preferably, within 30 sec) because stress-induced cardiac abnormalities dissipate rapidly upon termination of exercise. Therefore, images cannot be acquired quickly enough if the treadmill is located remotely from the imaging system.

Furthermore, requiring the patient to walk any distance immediately following maximal exercise may produce dizziness and risk of falling even in relatively healthy patients. EXCMR’s totally MRI-compatible treadmill solves these problems by placing the treadmill in the MRI suite, minimizing the time between end of exercise and imaging to maximize safety as well as accuracy of the test.